Digestion of organic waste is utilised in a plurality of processes for reducing volumes of waste and simultaneously producing biogas. In digestion, the organic waste is mixed with a culture of bacteria and is then digested under anaerobic conditions. In digestion, the organic waste is decomposed, thus producing biogas, which essentially consists of methane and carbon dioxide, and digested sludge.
U.S. Pat. No. 4,652,374 in the name of Cohen discloses a method of digesting organic waste in two steps. The solid organic waste is ground in such a manner that 80% has a particle size of 0.25-1.5 mm. Hydrolysis/acidification takes place in a first step. The liquid from the first step is separated and supplied to a second step where the main production of methane takes place.
U.S. Pat. No. 4,386,159 in the name of Kanai discloses a method of digesting organic waste matter with a certain ratio of carbon to nitrogen. The organic waste matter is ground to a juice-like liquid and is then mixed with a bacteria-containing sludge in a tank. Then the digestion is allowed to proceed in the tank without agitation for about 5-7 days.
It is a disadvantage in the above processes that the production of biogas is inefficient and that the biogas therefore will be expensive.